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Astron. Astrophys. 323, L25-L28 (1997)

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1. Introduction

Water vapour, one of the first molecules observed in radio astronomy, was discovered in 1962 by Cheung et al. through the maser emission of its 616 -523 rotational transition at 22 GHz. Since then, many lines of H2 O have been observed at submillimeter wavelengths from the ground. However, all these lines are masering which renders the interpretation of the data difficult (Cernicharo et al. 1990; Menten & Melnick 1991). Recently, Cernicharo et al. (1994, 1996) and González-Alfonso et al. (1995) have mapped the H2 O 183.3 GHz transition in three star-forming regions (Orion-IRC2, HH7-11, and W49N) providing the first evidence that the distribution of water vapour is extended and that the H2 O abundance can be larger than 10-5 in warm molecular clouds.

In this Letter, we present further evidence that water vapour is extended in molecular clouds based on the detection of far-infrared H2 O transitions seen in absorption against the continuum of the SgrB2 molecular complex.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998